基于古地磁与~(230)Th定年的西沙西科1井乐东组生物礁沉积年代的初步研究

生物礁是重要的自然资源,在全球气候变化与碳循环中扮演了重要角色.磁性地层学是建立年代框架的有效手段,但是,由于生物礁沉积物中天然剩磁强度弱,南海地区生物礁的磁性地层学研究尚未很好展开.为此,本文利用西沙群岛西科1井乐东组生物礁沉积样品进行了详细的岩石磁学和磁性地层学研究.结果显示,西沙群岛乐东组记录了布容正极性时、奥杜维尔正极性时和松山负极性时.通过对比已有的钻孔资料,本文认为应基于岩石地层特征这一标准将西沙地区的乐东组埋深予以统一.在此基础上,综合磁性地层与~(230)Th定年结果,本文将乐东组的底界限定在~2.0 Ma.

Magnetostratigraphy and 230Th dating of Pleistocene biogenic reefs in XK-1 borehole from Xisha Islands,South China Sea

Biogenic reefs and carbonate platforms are valuable natural resources, which play a role in modulating global climate and carbon cycle through biological processes. In this study, by combining paleomagnetic data and ²³⁰Th dating with lithostratigraphic characters for the XK-1 biogenic reef sequence from the Xisha Islands, northern South China Sea, new insights into the regional geochronology since the early Pleistocene are obtained. The main results are as follows:(1) Paleomagnetic findings suggest that the sequence records the Brunhes normal chron and the Matuyama reverse chron, including the Jaramillo and Olduvai normal subchrons. (2) The studied biogenic reef sequence named as the Ledong Formation has two subunits, i.e., upper and lower sections, which can be correlated to the middle and late Pleistocene, and the early Pleistocene, respectively. By comparing our results with those for previous long cores from the Xisha area, we conclude that the previous debate on the position and age of the Ledong Formation around the Xisha area mainly arose from inadequate absolute dating and use of different criteria for lithostratigraphic boundary determination. After determining the lower lithological boundary of the Ledong Formation, we set the age of the boundary as~2.0 Ma.